Power plants with boilers generally experience discontinuous accumulation of operating waste water. In particular in the case of natural-circulation and forced-circulation boilers (drum boilers), there is a need to remove the impurities from the circuit via the drains on the boiler drum.
In the case of power plants having drum boilers, the boiler waste water accumulating during plant operation can generally be divided into two groups. “Clean” waste water, that is to say water from the drains in the steam region (drains of the superheater heating surfaces in the boiler), can be directly reused in the water-steam circuit by virtue of the chemical composition of the waste water. In the case of “dirty” waste water from the drains in the “water region” (drum drains), in which the chemical composition of the waste water does not permit direct reuse in the water-steam circuit, reprocessing is provided prior to reuse.
The “cleanliness” of the water from the drains in the steam region is due to the fact that, upon separation into a water phase and a steam phase in the boiler drum, any impurities remain in the water phase and the steam is “clean” as it leaves the drum.
In that context, it is desirable, with a view to reducing operating costs, to reduce the accumulation of waste water and/or to reuse this operating waste water. This is opposed, on the other hand, by corresponding difficulties in the construction of the power plant, such that it was generally not realistic, with the technical options known hitherto and with respect to the economic viability of the plant as a whole, to minimize this accumulation of waste water.
The operating waste water that accumulates is therefore generally simply collected and then discarded wholesale, i.e. finally poured into the general waste water system. In that context, it is generally necessary to process the waste water beforehand according to the legal boundary conditions. It must be assumed, on the basis of the foreseeable further tightening of environmental protection regulations, that in the future a reduction in the quantity of waste water will be legally imposed, or that the discharge of waste water, including processing, is made so expensive that a reduction in the quantity of waste water becomes economically realistic. In addition, this operating waste water has to be replaced with (completely desalinated) circuit water, which is costly to prepare, thus giving rise to investment costs (for a sufficiently large complete desalination plant) on one hand and to operating costs on the other.
Furthermore, the discharge of this generally very hot operating waste water of the boiler represents a significant energy loss which is evidenced during operation by a loss of efficiency and of power of the power plant, or during shutdown by more rapid cooling of the boiler. The latter is damaging, in particular in relation to possible start-up times and reduction in the service life of thick-walled boiler components.
Hitherto, the boiler waste water—so that it would not necessarily have to be discarded—was collected (partially separated into clean and contaminated waste water), cooled, where necessary reprocessed by means of a condensate purification plant, stored and fed back to the circuit in a greater or lesser quantity. Hitherto, the energy contained in the waste water was given off to the environment, unused (in part via special cooling systems).
WO 2007/077248 discloses at least how the quantity of water that accumulates and is to be cleaned can be reduced, this being done by the clean boiler waste water being intercepted by collection pipes assigned to individual pressure stages and being fed directly back to the respective evaporators. Thus, water that accumulates, in particular during shutdown, can be stored in the boiler itself and no external storage capacity is required.